The invention describes means to stabilize free electron lasers.
Free electron lasers include a charged particle accelerator, a straight section, means to inject a relativistic electron or positron beam from the accelerator into the straight section, an undulator mounted coaxially along said straight section, which undulator periodically modulates in space the acceleration of the electrons passing through it thereby inducing the electrons to produce a light beam practically colinear with the axis of the undulator, and an optical cavity defined by two mirrors mounted facing each other on either side of the undulator to permit the circulation of the light thus emitted, so that a laser amplification occurs when the period of said circulation of light coincides with the period of passage of the electron packets and the optical gain per passage exceeds the light losses which occur in the optical cavity.
We note in passing that particle accelerators operate as well with electrons as with positrons which are the homologues of electrons with a charge of the opposite sign. Both kinds of particles belong to the class of charged leptons. The light is herein defined as electromagnetic radiation including not only the visible range but also the range of neighboring wavelengths in the near and far infrared as well as in the ultraviolets (so-called "vacuum ultraviolets") and even x-rays. Finally, we have spoken above of the coincidence between the circulation period of light and the period of passage of the electrons. Of course, said coincidence can be obtained not only when the periods are equal, but also when they are full multiples of each other or both full multiples of the same time span.
Free electron lasers present significant advantages over the lasers presently used in the industry. Actually, free electron lasers are expected to provide a power output superior to the power available from the traditional lasers. They present, in addition, the possibility of being tuned continuously over a wide range of wave lengths.
However, the first experiments showed certain erratic aspects of laser operation.